Why New Fish Die in the First Week (and How to Prevent It)
Losing a new fish is heartbreaking. Here are the most common causes — and the simple habits that keep new fish healthy.
Beginner 5 min read
There’s nothing worse than bringing home a beautiful new fish and finding it unwell a day or two later. The good news: almost every early loss comes down to a handful of causes, and all of them are preventable.
1. “New tank syndrome”
What happens: fish are added to a tank that was set up only hours or days earlier. Their waste quickly builds up as ammonia, and there aren’t yet enough good bacteria in the filter to break it down.
How to prevent it:
- Run a new tank with its filter for 2–4 weeks before adding fish (this is called cycling).
- Start with a few hardy fish, then add more slowly.
- Never wash filter sponges under the tap — rinse them in old tank water.
Our beginner’s checklist explains cycling step by step.
2. Temperature shock
What happens: the water in the bag and the water in your tank can be several degrees apart. Pouring a fish straight in shocks its system.
How to prevent it — acclimatise properly:
- Switch off the tank light (bright light stresses new arrivals).
- Float the closed bag in your tank for 15–20 minutes so the temperatures match.
- Open the bag and add a small cup of tank water every 5 minutes, three or four times.
- Gently net the fish into the tank. Try not to pour shop water into your aquarium.
3. The trip home
What happens: bags left in a hot car or in direct sun heat up fast — especially in a Pune summer — and oxygen in the bag runs low over time.
How to prevent it:
- Make the fish shop your last stop and go straight home.
- Keep the bag out of direct sunlight, somewhere shaded and steady.
4. Overfeeding
What happens: it’s tempting to feed new fish a lot. Uneaten food sinks, rots and pollutes the water — a new tank can’t handle it.
How to prevent it: feed a small pinch once a day for the first few days, only what they eat in about 2 minutes. New fish often don’t eat much on day one, and that’s normal.
5. Overcrowding
What happens: too many fish added at once overwhelms the filter and the fish compete for space.
How to prevent it: add fish a few at a time, spaced a week or two apart. Tell us your tank size and we’ll suggest a sensible number.
6. Tankmates that don’t get along
What happens: some fish nip fins, some are territorial, and some simply need different water.
How to prevent it:
- Don’t keep goldfish with tropical fish.
- Keep one male fighter fish (betta) per tank, and avoid fin-nippers around long-finned fish.
- Keep schooling fish, like tetras, in groups so they feel secure.
7. Untreated tap water
What happens: municipal tap water contains chlorine to keep it safe for people — but it damages fish gills.
How to prevent it: add a water conditioner every time you add new water to the tank.
Something doesn’t look right?
Watch for fish gasping at the surface, clamped fins, hiding, not eating, or white spots. If you notice any of these, test or change some water first — most problems start with water quality.
And you’re never on your own: send us a short video and a description on WhatsApp, or come and talk to us at the store. We’re always happy to help you work out what’s going on.